If the electrical system of your heart is diseased, injured or not developed in the usual way, you may experience an abnormal heart rhythm. This abnormal beating (rhythm) is called an arrhythmia. Almost all of the cells of your heart are capable of starting or causing a normal or abnormal heart rhythm (arrhythmia). Arrhythmias are generally classified into groups.

Rapid heart rhythms are called tachycardias. Tachycardia is a heart rhythm that beats more than 100 times per minute. A tachycardia may begin in the atria (upper heart chambers), the sinoatrial node (AV node) or in the ventricles (lower heart chambers). Because the ventricles are responsible for pumping blood to the lungs and the body, fast heart rhythms in the ventricles are generally more serious.

• Sick sinus syndrome is a type of bradycardia. Bradycardia is a slow heart rate, generally less than 60 beats per minute. When you suffer from bradycardia, your sinus node, which initiates each normal heartbeat, fails to trigger the electrical signals as it should. In other words, it may not start the heartbeat often enough, it may skip or miss some of the heartbeats, or it may send too many heartbeats. The heart may beat too slowly, or it may alternate between too slow and too fast. Common treatment for this is a pacemaker.
   
• Heart block or AV block is a condition in which your heart rhythm is slowed down because your heart skips one or more beats.  When you suffer from heart block, the electrical signal is interrupted as it travels from the upper chambers of the heart (atria) to the lower chambers (ventricles). This interruption results in a bradycardia (slow heart rhythm). Common treatment for this is a pacemaker.
   
• Supraventricular tachycardias (SVT) are fast heart rhythms that originate in the upper chambers (atria) of the heart. You can suffer from several different types of SVT. Common treatments for SVT are medications and radio frequency ablation.
   
• Premature atrial contractions (PACs) are early, extra beats originating in the upper chambers of the heart. Usually no treatment is required for PACs.
   
• AV node reentrant tachycardia (AVNRT) is the most common type of supraventricular tachycardia. In this tachycardia, an extra pathway exists within the AV node. If an electrical signal enters this pathway, it may start traveling in a circular pattern within the AV node causing a very rapid heartbeat. A common treatment for this is radio frequency ablation.
   

Wolff-Parkinson-White Syndrome is a type of supraventricular tachycardia caused by an extra pathway or connection between the atria and the ventricles. The electrical signal from the top of the heart may pass through this connection instead of the AV node, which normally slows down the signal before it is transmitted into the ventricle. This syndrome may cause tachycardia or some specific abnormalities (what kind of abnormalities?) on an ECG .